Directed evolution has emerged in the past decade as the most powerful method to improve biocatalysts.[1] For instance, the enantioselectivity of a D-hydantoinase could be reversed leading to a substantially improved process for the synthesis of optically pure L-amino acids.[2]
Reetz and Jaeger were able to increase the enantioselectivity of a lipase from Pseudomonas aeruginosa towards a chiral carboxylic acid (2-methyl decanoate) first from E=1.1 (wildtype) to E=11.[3] Using a combination of a broad range of molecular biology methods they were finally able to identify a variant with practically useful selectivity (E=51).[4] Similarly, we were able to increase the enantioselectivity of an esterase from Ps. fluorescens (PFE) towards 3-phenylbutyric acid from E=3.5 to E=12 by combining error-prone PCR with saturation mutagenesis.[5]
Successful directed evolution experiments strongly depend on several aspects: The identification of desired variants exhibiting an increased enantioselectivity strongly depends on the high-throughput screening method used, as the selectivity determined using a surrogate substrate (i.e. a p-nitrophenyl ester) can differ significantly from the true substrate (i.e. a methyl ester) which can lead to false positive variants. In addition, it is assumed that the random introduction of mutations does not significantly affect other properties of the biocatalyst and its production in the microbial host. Another aspect is the location of productive mutations. Directed evolution experiments often lead to variants, in which effective mutations were far from the active site region, although they affected substrate specificity or enantioselectivity and the question whether closer mutations are better has been already addressed in literature.[6]
The hydrolase-catalyzed resolution of the acetate of 1a is very challenging, as this secondary alcohol has only small differences in the size of its substituents. In accordance to the ‘Kazlauskas rule’[7] it is converted by lipases or esterases only with low to modest E-values.
The (R)-alcohol can be used for the synthesis of (−)-akolactone A, a cyclotoxic butanolide[8], pancrastatin, an antitumor alkaloid[9], chiral cyclopropane-based ligands[10] and for the large-scale production of (R)-enzyl 4-hydroxyl-2-pentynoate.[11] Both pure enantiomers of 1a were employed in the preparation of enantio- and diastereomerically pure allylboronic ester by Johnson rearrangement, leading to enantiopure homoallyl alcohols.[12] Other examples include the synthesis of 3-bromo-pyrrolines from α-amino allenes[13], optically active bicyclic ligands used for the synthesis of HIV protease inhibitors[14] or key intermediates of αvβ3 antagonist (osteoporosis).[15] One example for an application of the (S)-alcohol is the preparation of chiral cyclic carbonates.[16]
Previously, we investigated >100 hydrolases for the kinetic resolution of 1b[17] and the best enzyme was the esterase from Pseudomonas fluorescens (PFE) recombinantly expressed in E. coli.[18] (SEQ ID NO:1). But detailed analysis of the reaction time course revealed, that the enantioselectivity considerably dropped during the hydrolysis of the acetate (Table 1) and complete conversion was observed leading to racemic alcohol 1a. A similar effect was described in a patent for an esterase from Pseudomonas glumae.[19] In the hydrolysis of the corresponding butyrate, the initial value was E ˜30, but then dropped too at higher conversion making a high yield resolution impractical. Only Nakamura and coworkers reported acceptable enantioselectivity, but the small-scale resolution suffered considerably from the use of large amounts of lipase Amano AH and very long reaction times (2 d).[20] The alternative asymmetric enzymatic reduction of the ketone is hampered by the very low enantiomeric excess as shown for a NADPH-dependent alcohol dehydrogenase from Lactobacillus brevis affording the (R)-alcohol with 60% ee or a NADH-dependent Candida parapsilosis carbonyl reductase yielding the (S)-alcohol with 49% ee.[21] In addition, the 3-butyn-2-one is rather unstable with high risk for thermal decomposition restricting the large-scale reduction, even if a highly selective reductase is available.